

	Mechanics På svenska

Doctoral defense

Suspensions of finite-size rigid particles in laminar and turbulent flows


Defendant	Main Advisor	Extra Advisor	Date
Walter Fornari	Luca Brandt	Minh Do-Quang	2017-12-15

Opponent
Jérémie Bec, CNRS Laboratoire Lagrange UMR7293

Evaluation committee
Micheline Abbas, Génie des Interfaces & Milieux Divisés, Hydrodynamique et Transport Nils Erland Haugen, NTNU Norwegian University of Science and Technology Martin Trulsson, Lunds Universitet

Abstract

Dispersed multiphase ?ows occur in many biological, engineering and geophysical applications such as ?uidized beds, soot particle dispersion and pyroclastic ?ows. Understanding the behavior of suspensions is a very di?cult task. Indeed particles may di?er in size, shape, density and sti?ness, their concentration varies from one case to another, and the carrier ?uid may be quiescent or turbulent. When turbulent ?ows are considered, the problem is further complicated by the interactions between particles and eddies of di?erent size, ranging from the smallest dissipative scales up to the largest integral scales. Most of the investigations on this topic have dealt with heavy small particles (typically smaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior of suspensions of ?nite-size particles (particles that are larger than the smallest length scales of the ?uid phase). In the present work, we numerically study the behavior of suspensions of ?nite size rigid particles in di?erent ?ows. In particular, we perform direct numerical simulations using an immersed boundary method to account for the solid phase. Firstly, the sedimentation of spherical particles slightly smaller than the Taylor microscale in sustained homogeneous isotropic turbulence and quiescent ?uid is investigated. The results show that the mean settling velocity is lower in an already turbulent ?ow than in a quiescent ?uid. By estimating the mean drag acting on the particles, we ?nd that non stationary e?ects explain the increased reduction in mean settling velocity in turbulent environments. Moreover, when the turbulence root-mean-square velocity is larger than the terminal speed of a particle, the overall drag is further enhanced due to the large particles cross-?ow velocities. We also investigate the settling in quiescent ?uid of oblate particles. We ?nd that at low volume fractions the mean settling speed of the suspension is substantially larger than the terminal speed of an isolated oblate. This is due to the formation of clusters that appear as columnar-like structures. Suspensions of ?nite-size spheres are also studied in turbulent channel ?ow. We change the solid volume and mass fractions, and the solid-to-?uid density ratio in an idealized scenario where gravity is neglected. The aim is to independently understand the e?ects of these parameters on both ?uid and solid phases statistics. It is found that the statistics are substantially altered by changes in volume fraction, while the main e?ect of increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (? 1000) the solid phase decouples from the ?uid, and the particles behave as a dense gas. In this ?ow case, we also study the e?ects of polydispersity by considering Gaussian distributions of particle radii (with increasing standard deviation), at constant volume fraction. We ?nd that ?uid and particle statistics are almost unaltered with respect to the reference monodisperse suspension. These results con?rm the importance of the solid volume fraction in determing the behavior of a suspension of spheres. We then consider suspensions of solid spheres in turbulent duct ?ows. We see that particles accumulate mostly at the corners. However, at large volume fractions the particles concentrate mostly at the duct core. Secondary motions are enhanced by increasing the volume fraction, until excluded volume e?ects are so strong that the turbulence activity is reduced. The same is found for the mean friction Reynolds number. The inertial migration of spheres in laminar square duct ?ows is also investigated. We consider dilute and semi-dilute suspensions at di?erent bulk Reynolds numbers and duct-to-particle size ratios. The highest particle concentration is found in regions around the focusing points, except at very large volume fractions since particles distribute uniformly in the cross-section. Particles also induce secondary ?uid motions that become more intense with the volume fraction, until a critical value of the latter quantity is reached. Finally we study the rheology of con?ned dense suspensions of spheres in simple shear ?ow. We focus on the weakly inertial regime and show that the suspension e?ective viscosity varies non-monotonically with increasing con?nement. The minima of the e?ective viscosity occur when the channel width is approximately an integer number of particle diameters. At these con?nements, the particles self-organize into two-dimensional frozen layers that slide onto each other.
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